Removing mill scale from iron



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6/800? lbs:

United States Patent 3,073,726 Patented Jan. 15, 1963 today when thisduration has been reduced to a few 3,073,726 months as a result oftechnical advances.

REMOVING MILL SCALE FROM IRON Herbert Manfred Freud, dit Jean Frasch, 5Blvd. du Sud-Est, Nanterre, France No Drawing. Filed Mar. 1, 1954, Ser.No. 413,450 Claims priority, application France Mar. 5, 1953 6 Claims.(Cl. 1343) The present invention relates to the treatment of metalsurfaces. More particularly, the present invention relates to a processand composition for treating the surface of steel articles, e.g. in theform of plates, to provide for improved adherence of paint to thesurface of the article to protect the same from the effects ofcorrosion.

It is known that mill scale, formed on steel in the course of rolling,forging or stamping under heat, constitutes an extremely harmful layerpreventing good adherence of paint or other protective coating on themetal of the base, thus risking exposure of the metal base todestruction by corrosion.

The process of removing the mill scale from small tion of hydrogen bythis attack (Me+HX=MeX+H) 5 which, accumulating under a certainpressure, starts to raise and detach the mill scale which falls to thebottom of the vat.

The addition to the acid bath of agents for inhibiting the corrosion ofthe metal serves the purpose of limiting the speed of attack of thesteel which might be too extensive and produce a deterioration of theobjects, either by a substantial diminishing of the material, or as theresult of the so-called hydrogen embrittlement which is imparted to thepiece, and which is manifested by a substantial reduction of the elasticproperties of the metal.

When large objects are to be de-mill scaled, either of fixed type, suchas bridges, pylons, and gasometers, or mobile type, such as a ship, forexample, there is actually available only two methods, as follows:

(a) Either sand blasting or mechanical brushing, operations which are,very costly, very dangerous and impossible to carry out in certaincases, as, for example, in the vicinity of gear mechanisms (where veryfine sand in penetrating the mechanism may scratch and adhere everywhereto the same) or in a petroleum refinery (where sparks produced by thesand blasting may start a fire). Moreover, and chiefly with respect toclosed places, e.g., the interior of a ship, a great problem is involvedin the removal of all the sand, which sticks to the walls and therebyinterferes with good adherence of paint, or

(b) Natural exposure of the objects, and in particular of sheet metal,to corrosive atmospheric agents (especially at the seashore) for one ortwo years, which pro vides for penetration through the pores of the millscale of the oxygen of the air and the atmospheric humidity for thepurpose of attacking the iron underlying the mill scale which theytransform to rust. The rust, occupying a volume 13 times greater thanthe metal from which it is formed, exerts a pressure on the mill scalewhich is so great that it is mechanically detached, in the manner of thehydrogen in the acid scouring bath.

De-mill scaling by natural exposure presents two major difficulties:

(1) It requires an extremely long duration, one to two years, a periodwhich was not inconvenient some years ago when the construction of aship, for example, required two to three years, but which is a greathandicap (2) It involves the useless destruction of a large part ofsound metal, because the rust does not form uniformly over the wholesurface, but rather by regions often hollowed out in depth, so as toform what are called in technical terms cankers.

Another method which is theoretically valid consists in sprinkling thesheet metal or other objects coated with mill scale with a mineral acidor sea water in order to de-mill scale the same. Even if by this processone can accelerate the speed of de-mill scaling with respect to thenatural exposure method, the experience of the inventor (as well asother numerous technicians who have tried this process before him)proves that this method is either of little elfect, or is harmful to thesheet metal for the fol lowing reasons:

(a) When an aqueous solution of a strong acid or sea water is applied ona smooth metal wall which is vertical or forms a ceiling, the solutiondue to its fluidity flows toward the bottom under the action of gravity.It remains adherent to the mill scaled metal only as a thin film havinga thickness of the order of a few microns, containing a quantity of acidor salt which is insuliicient to effect electrolytic and hydrolyticreactions. If this operation is effected in the free air, in windyperiods the thin liquid film is immediately swept off, in sunny periodsit is immediately evaporated, and in rainy periods it is immediatelywashed oil, and since of the 365 days of the year it is either windy,rainy or sunny at least during 300 days, this operation has provedillusory.

(b) In the hollows and on the flooring, on the other hand, there is aconsiderable accumulation of free acid or salt which favors the de-millscaling operation, but which however may cause partial dissolution orstrong corrosion of the sheet metal in places which may prove extremelydangerous, especially between two plates which are riveted, welded orbolted.

(c) The addition of wetting agents in the acid or saline solutions inorder to favor the penetration of the latter between the mill scale andthe sheet metal has the elfect of reducing the surface tension of thesesolutions to assist their spreading out, thereby to diminish thethickness of the liquid film, and as a result reducing the speed of thede-mill scaling process.

(d) The addition of thickening agents (such as gelatin or other sizingmaterials) to the acid or saline solutions has a double result, on theone hand to provide for the increase in thickness of the film (andthereby the quantity of acid and thus the speed and the effectiveness ofthe demill scaling procedure), and on the other hand to increase theover potential of the hydrogen (and thereby retard the electrolyticaction of the mill scale iron pile," this word "pile being used in thepresent specification to designate what is usually called cell, and thusthe speed and the effect of the de-mill scaling operation). There is,therefore, no advantage in adding any thickening agent with an adhesivebase to the acid or saline solution. On the contrary, it is quitedifficult to paint on a surface on which gelatin or adhesive materialshave been applied due to their tendency to absorb water vapor, whichraises up the paint which is applied.

It is an object, therefore, to provide for de-mill scaling metalsurfaces by a process which avoids the above mentioned disadvantages.

Another object of the present invention is to provide a de-mill scalingmaterial which can be applied on all steel or cast iron objects andwhich can effectively and rapidly remove the mill scale layer therefromwithout an appreciable attack on the metal of the base.

Other objects and advantages will become apparent from the followingdescription and the appended claims.

In particular, the invention concerns the provision of a paste materialhaving a viscosity suflicient to enable the paste to be applied with abrush or with a spray gun (with the aid of a vessel under pressure) onvertical members or on a ceiling, the paste preferably having athickness of between 0.01 and several millimeters without running ofi,that is, such as to remain at the same thickness as long as it is notremoved. The paste, in accordance with the invention, has a suificientlystrong action to provide for the de-mill scaling operation in arelatively short period of time, and of such composition to keep thesheet metal protected against all attack and irregular corrosion of thecanker type, as well as all corrosion in the interstices between twoplates which are riveted, welded or bolted.

The product provided in accordance with the invention is a pastematerial which comprises as essential components one or more oxidizinganions and an inert pulverized mineral charge (incapable of beingattacked, particularly by acids) such as silicates.

The mineral charge in accordance with the invention is preferablycomposed of an insoluble silicate or of silica itself, such as clay orkaolin (aluminum silicate), talc (magnesium silicate), bentonite orground shells (silica and calcium silicate). The mineral charge shouldbe finely ground and moistened with water, in order to obtain a greasypaste so that it can be spread out in the manner of an oil paint. Groundquartz or silica which is entirely dehydrated is not suitable, since apaste made therefrom has the tendency to form lumps.

The function of the mineral charge utilized in the paste formed inaccordance with the invention is not that of a thickening agent for theacid solution, but that of a porous vehicle or vessel, that is, to serveas a support for osmotic phenomena.

The function of the silicates contained in the charge conforming to theinvention can be compared to that of a porous vessel in which a plant isgrown. The porous vessel serves for the exchange of humidity and airbetween the roots of the plant (and the surrounding earth) and thesurrounding atmosphere resulting from the osmotic pressure which isestablished through the vessel.

In accordance with this concept, the inventor has been able to show thatthe swelling colloidal charges capable 4a of forming a true gel withwater, such as bentonite, are preferable to others due to theirparticular capability of serving as a porous vessel, that is, to absorbeither the water provided by the atmospheric humidity, or othermoistening agent, in swelling, according to the expression used inmineral chemistry, and the present invention preferably utilizes suchcharges.

Another property of such swelling colloidal charges is the possibilityof obtaining the same viscosity of the paste with ditferent proportionsof the liquid-solid components, according to the pH of the medium fromwhich the first gel has been formed. Thus, for example, when thebentonite has been added to a sulfuric a cid solution in order to obtaina given viscosityfit willbe necessary to have 35% of bentonite and 65%of acid solution. On the other hand, if initially a gel 0 entontte isformed with water and the sulfuric acid is then added in order to obtainthe same viscosity of the paste, and the same final proportion of thesulfuric acid with respect to the water, it will be necessary to have25% of bentonite and 75% of acid solution (water and acid). It is thuspossible to obtain the same viscosity in accordance with the method ofpreparing the paste with a lesser amount of the charge and increasedamount of the ionizing and de-mill scaling dilute acid, and to vary asdesired the viscosity of the paste by small additions of acid, water andvarious solutions, knowing that the addition of acid of low pHfacilitates the liquefying of the gel, while water or solutions ofhigher pH facilitate the thickening of the paste.

The proportions of the components of the paste, ac-

cording to the invention, can be varied within very great limits. Aprimary condition is the obtaining of such a viscosity that the pastecan be applied on a ceiling or on a vertical wall without running, in alayer having a thickness of the order of 1 mm., but the proportions ofthe active ingredients are not critical, having effect only on the speedof action of the paste material.

The condition relative to the pressure-of the oxygen (p0 can beexpressed by saying that these oxidizing anions have an rH greater than41 and an r0 less than 0 by virtue of the following relations:

By way of an example of such oxidizingagents there may be mentione MnO;within the limits of ClO" at all pH f 10 at a pH between 1 and 1.5

; cl'2O7 at a pH between 1 and 1 i etc.

The rH-value is the logarithm, to the base 10, of the reciprocal of thehydrogen pressure which would produce the same electrode potential asthat of a given oxidation-reduction system, in a solution of the samepH-value. The greater the oxidizing power of a system, the greater therH-value.

The pH of the paste can be whatever at the moment its rH or r0 satisfiesthe condition described above. It can clearly be located between 0 and10.

Certain oxidizing agents, such as hypochlorites or periodates, produceupon reduction salts which are good conductors and which enable thepaste to continue its action, but certain others such as KMnO, produceinsoluble products which make the iron passive and interrupt the actionof the paste.

In this case, the invention provides for the addition to the paste of asalt or an acid soluble in a liguid which is a good conductor whichavoids this passive action and has, besides, the advantage of aiding theformation of electrolytic piles of the type described below, in the casewhere the mill scale has a certain porosity. These salts or acidspreferably have the following properties:

(a) Ionic activity greater than 0.01 expressed in the usual manner asconcentration of the ion in solution in grams per liter.

(b) Reactivity on the iron (iron electrode potential in their aqueoussolution less than 0).

(c) rH between 27 and 41.

They can be sulfates, acetates, pyrophosphates, of metals, preferablyalkaline metals, for example,

of rH of the permanganate KMnO, are given below at different pH:

pH between 1 and 11 pH r0 rH in 1o 1! so a 10 47.5 s 10' 45 P9 10 42.5

The first solution is practically destroyed at the end of a day or two,the third has a duration of more than three months. Therefore, it ispreferable to avoid the prolonged presence of strong acid ions.

However, this condition tending to do away with the use of strong acidsin the pastes is valid only for reasons of transport and storage. It isnot significant with respect to the activity of the paste. As aconsequence, the procedure which provides for the introduction in thepaste at the time of its employment of a strong acid or otherelectrolyte corresponding to the properties specified above, comeswithin the scope of the invention. This procedure, in fact, avoids the risk of destruction of the paste, since the paste is used as soon as theaddition is made.

As indicated above, the proportions of the components can be variedwithin very great limits. According to the concentration of theoxidizing agent, the paste is more or less active, but its actionremains the same and therefore the proportions are not critical.

In the same way in accordance with the proportion of its constituents,the paste is more or less fluid, and more or less adapted to varioustypes of de-mill scaling operations. The conditions of employmentthemselves being quite varied, and the layers of mill scale being verydifferent in composition and thickness, the specifying of proportions ofthe components does not have very im portant significance.

However, it may be noted that the proportion of water of aqueoussolution in the paste will be regulated as a function of the viscositysought, which depends on the instruments used for applying the paste.This can be determined by one skilled in the art, and according to theinstrument employed, i.e. a spray gun, a brush, painter knife, it willbe necessary to adopt a certain viscosity from which the necessaryproportion of the liquid can be determined. Besides, the user can alwaysincrease the fluidity at the time of use by the simple addition ofwater.

With respect to the proportion of the oxidizing agent, this can varybetween 1% and 20% with respect to the total weight of the paste,according to the oxidizing agent adopted and the activity of the pastesought. Also, the electrolyte can be employed in the proportion of 1% to30% according to its conductivity and its eventual particular activityin the electrolytic pile. But it is to be noted that these proportionsare not critical.

In the case where the partial pressure of the oxygen of the oxidizinganions is less than 1 atmosphere (rH less than 41), the mere presence ofoxidizing anions is not always suflicient, and the invention providesthen for the addition of non-reducing mineral acids having a pH lessthan 2.5, and in particular, sulfuric acid. It appears that, in thiscase, the mechanism of the reaction is as follows:

In the course of the penetration of the acid (and in particular sulfuricacid) through the mill scale, it reacts in part with the layer of FeObetween the metal and the mill scale with the formation of ferroussulfate and in part with the iron itself with the formation of hydrogenand ferrous sulfate equally. It is initially due to this formation offerrous sulfate and then to its oxidation to ferric sulfate and rust bythe action of the oxidizing agent that the adherence of the mill scaleto the metal is diminished, and the de-mill scaling operation renderedpossible.

In fact, if by rapid oxidation of the iron or of the ferrous salts, suchas in the presence of an energetic oxidizing agent, there is obtainedthe variety Fe o xH oa or goethite, there is produced, on the contrary,by a moderate oxidation (such as by the simple action of air and theacid or saline solutions) the variety Fe O xH O'y or lepidocrocite whichreacts with FeO or the ferrous sulfate, forming the ferrosoferric Fe OxH O or the mill scale hydrate. In contrast to the variety of red rustor goethite Fe O xH Om which occupies a volume 13 times greater than themetal from which it has been formed, the variety of black rust orferrosoferric Fe O xH O occupies a volume hardly equal to or triple thatof the metal. The force of expansion and thereby the ease of raising upthe mill scale is then greatly increased by the formation of thegoethite instead of the ferrosoferric oxide, and the presence ofoxidizing ions in the paste, produced a rapid oxidation of the ferroussulfate and the destruction of the hydrogen, permits this result to beachieved.

The acid utilized can be any mineral acid having a pH less than 2.5,with the exception of an acid capable of reducing the oxidizing anionpresent in the paste (such as HBr, H SO H PO H PO etc.). As a practicalmatter, the acid which seems to be of the greatest interest, at leastfrom the point of view of economy, is sulfuric acid, at a concentrationvarying between 5% to 50% .with re spect to the water contained in thepaste.

The oxidizing anion can be of any type, so long as it is reduced bynascent hydrogen at ordinary temperature and can oxidize the ferroussulfate formed by the action of H on the iron and eventually the layerof FeO, as for example the anions CrO Cr O' MnO N0 N0 C10 1 C105, etc.Those anions are preferable which aresiisceptible of changing color whenthey pass from the oxidized state to the reduced state, such as theanion CI'gO-f' which is orange yellow and which, by reduction, istransformed to the blue green cation Cr+++, or the anion MnOywhich isdark brown and which by reduction is transformed into the pale rosecation Mn++.

The paste, which in the absence of these oxidizing anions, is white orcolorless, is strongly colored in the presence thereof, either to anorange yellow in the case of the presence of Cr O- ions, or to darkbrown in the case of the presence of MnO and when it is applied on themill scaled metal it shows the same colorations. In the degree that theacid penetrates through the pores of the mill scale, it attacks theferrous oxide and the underlying iron, which it transforms into R250,and liberates hydrogen. These two products act as reducers of theoxidizing anion which they reduce to the state of a reduced cation, withclear change of color of the paste at that point. There will appear, infact, for example, in the case of a paste containing the 0 0,- ion, andwhich is orange yellow (more or less rapidly in accordance with theporosity and the thickness of the mill scale), points or regions whichare blue-green, which increase in proportion until the entire surface onwhich the paste has been applied is covered. The initial points whichchange color indicate the pores of the mill scale and the places wherethe acid begins to penetrate. If the change in color of the entiresurface does not mean that the acid has penetrated everywhere (becausethe change of coloration can also be produced by the diffusion of thehydrogen ion through the paste and the local reduction of the oxidizinganion), it nevertheless indicates that the acid has penetrated through agreat number of the pores of the mill scale and that it continues tocarry out its action of dissolution on the FeO and Fe under the millscale between two or more pores. It is then preferable to wait a fewdays after the change of color of the entire surface before proceedingto remove the paste by washing and to expose the thus Washed sheet metalto atmospheric agents for the purpose of forming rust.

It has beerijggnd, furthermore, that while the de-mill scalihg prdcesscan be carried out fairly well with the aid of a paste having thecomposition described above, even bgtteg re s ults qan be obtained,thatis, agreater speed of de-mill scaling can be achieyed if there isadded to the .Esisflahnacaffi ifiifi .5 .3, Nah i 'This can be achievedeasily by selecting, for example, Na cr O or K MnO as the bodycontaining the oxidizing anion.

The preferred proportions are of the order of 0.5 to 5% in weight withrespect to the paste, but these limits may be exceeded withoutdifiiculty.

This action is probably concerned with the mechanism of the formation ofthe rust by the pile effect between the metal and the mill scale. Theanion SO goes toward the iron anode and forms FeSO the cation K goestoward the mill scale cathode and forms KOH-t-H. The potassium hydroxidethus formed reacts on the ferrous sulfate in accordance with thereaction:

The ferrous hydroxide thus formed is more easily transformed to rust bythe oxygen of the air or the oxidizing agent contained in the paste(than the ferrous sulfate which requires for its transformation to rustboth oxygen and water) according to the reaction:

In the case of metals which are strongly mill scaled, the inventionprovides, in the case of the two types of oxidizing agents described,for leaving the above paste applied at least one week, washing it off,allowing the action of the atmospheric agents to continue for some timeuntil the piece is completely covered by rust, and then removing therust by any appropriate means.

These procedures will be more fully described below and explained bymeans of the following examples, it being understood that the examplesgiven below are set forth only for the purpose of illustration, and arenot intended to limit the scope of the invention in any way:

Example 1 A solution is first prepared containing:

600 gm. water 30 gm. KMnO 70 gm. (NH SO which is poured into a vessel towhich there is slowly added in order to form a uniform gel:

300 gm. of bentonite The pH of this paste is about 7, the r-H is 44, andits color is red.

The gel thus formed is applied to the surface to be demill scaled,either with the aid of a spray gun, or with a brush.

At the end of some time, there will be seen a change in color of thepaste which becomes maroon, this indicating that the KMnO has released apart of its oxygen for the function of the electrolytic and electronicpiles and is transformed to MnO The paste is allowed to react about 10to 15 days at the end of which time all of the KMnO has reacted and thepaste becomes useless. The paste is then removed by washing and finallycleaned off with a metallic brush to remove traces of the detached millscale.

In the case where the mill scale is thick and uniform, the process inaccordance with the invention is preferably carried out as follows:

The paste is applied on the mill scaled surface and it is allowed toreact for a few days. The minimum period which the paste is left on thesheet metal should be 48 hours, but it can be left on for several monthswithout having any accelerating or retarding elfect on the de-millscaling process. Thus, when the paste is applied on marine sheet metalin the course of storing the same in the storage depot, it will be lefton until the sheets are drawn out for use. When the paste is applied onthe bottom of a ship already built, for example, it will be left on 5 to6 days. At the end of 8 days it dries. It is then necessary to remove itby washing it with water, since it hinders the detachment of the millscale disintegrated by the rust which should form on the metallicsurface after several weeks of exposure to the air and which can beremoved either by a jet of water or a moistened sponge.

On the places where the mill scale has been relatively thin, it formsblisters. But in the places where the mill scale is sufficiently thickand uniform, the free acid takes a substantially longer time topenetrate to the metal (in passing by the layer of FeO), which thenreacts more slowly and produces liberated hydrogen which has a lowerpressure.

On the other hand the layer of mill scale, being thicker, requires agreater pressure of hydrogen than a thin layer in order to be raised up.It is for these two conjugate reasons that when a thick layer of millscale is met with, it is not detached from its iron base together withthe paste.

In fact, if dry paste is on the mill scale, it sufficiently hinders thepassage of humidity and atmospheric oxygen for finishing thetransformation of the ferrosoferric salts formed into ferric saltswhich, by hydrolysis, produces the rust. After washing off the paste,there is found on the sheet metal the mill scale which is visiblyintact, that is, it is black without a trace of rust. In reality, it isnot as adherent or as compact as it was initially. This will be easilyseen in scraping it with a knife or any blade, and in this way it israther easy to detach small chips of mill scale, which is not possibleto do on mill scale which is intact, that is, not treated by the paste.

The mill scale is then fissured and its adherence to the metal isconsiderably diminished. However, the ferrosoferric salts formed by theaction of the oxidizing and ionizing agents on the iron and the ferrousoxide do not .occupy a volume sutficient to exert an adequate pressureon the mill scale for the purpose of entirely detaching it.

When the paste has been removed by wash water, the oxygen of the air nolonger finds an obstacle to its passage through the pores of the millscale to the ferrosoferric salts, which is oxidizes to ferric saltswhich themselves, under the action of the humidity, hydrolyze to rust FeO xH O. The rust occupying a greater volume than the ferrous salts andthe iron from which it is formed detaches the semi-adherent mill scale.

The period of exposure of the sheet metal to atmospheric agents afterremoval of the paste is from 1 to 3 weeks, generally 15 days, and in allcases a period necessary so "that the entire surface is covered bypowdered rust, which can be removed by simple brushing with a metallicbrush.

Preferably after brushing, it is of advantage to utilize the lightcovering of rust spread over the entire surface, which may be phosphatedin accordance with known procedures, in order to obtain a betteradherence of the paint.

Example 2 There is first prepared a solution containing:

550 gm. of water 40 gm. of potassium permanganate 60 gm. of acetic acid50 gm. of sodium acetate This solution is poured into a vessel to whichthere is slowly added with agitation in order to form a uniform gel:

250 gm. of bentonite 50 gm. of kaolin The pH of this paste is about 4,the rH is 46.5 and its color is also red.

'Ihis paste, like the preceding one, acts by the formation both of anelectronic pile and an electrolytic pile. The electronic pile reducesKMnO to Mn0 as in Example l. The electrolytic pile, when the productspenetrate through the pores of the mill scale, reduces the red KMnO, towhite manganese acetate.

The application is carried out as in Example 1, and the period ofreaction varies also from 15 days to 1 month, including the time ofexposure to the air for rushing.

Example 3 There is prepared a solution composed of:

500 gm. of water 80 gm. of sodium hypochlorite 80 gm. of ammoniumbisulfate This solution is poured into a vessel, and there is slowlyadded with agitation in order to form a uniform gel:

250 gm. of bentonite 100 gm. of silica The pH of this solution is about6, the rH is 53 and the color is straw yellow.

The electronic pile functions then by the loss of the oxygen of thestraw yellow NaClO and its transformation to colorless NaCl. The pasteof straw yellow passes then to a white color. The mode of applicationand the action of this paste are the same as those indicated in Examples1 and 2.

Example 4 There is prepared at first a solution containing:

450 gm. water 170 gm. H 80 66 Baum 30 gm. Na Cr O' This solution ispoured into a vessel to which there is slowly added with agitation inorder to form a uniform gel:

350 gm. of bentonite The preparation is carried out at a temperature ofabout 40 C. which is easily obtained without heating by the simpleaddition of sulfuric acid in water. Obtained at this temperature, thegel is relatively fluid, but thickens by cooling. It is of yellow color.The gel thus formed is applied to the surface to be de-mill scaled,either with the aid of a spray gun or with a brush. After a few minutes,there will appear blue green spots which increase and multiply inaccordance with the porosity and thickness of the mill scale until theentire surface is covered. The time varies from a few minutes to a fewdays and cannot be specified, being determined solely by the particularsheet to be de-rnill scaled.

If the the pH of the paste is taken immediately after the entire surfacehas been colored blue green, it will be found that it is still less than2, that is, that all the acid has not reacted by penetration through thepores of the mill scale. If the sheet covered by the paste which hasturned green is left exposed for three to four days (three to four daysafter the complete change of coloration), and if the pH of the paste istaken, it will be found that it has gone up to about 4 in a practicallyuniform manner, that is, that all of the acid has already reacted, andthat it can be removed by washing without risk of having the wash waterremaining acid.

After washing it will be seen that the adherence of the mill scale onthe metal has been greatly diminished and that it can be removed byscraping. But this operation is long and expensive, and it is preferableto leave the thus washed sheet exposed to the atmosphere for 10 to 15days. Red rust appears after a few days and covers the entire surface.When this rust is sufiiciently abundant, it is usually brushed olf by ametallic brush or re moved by a paint knife by entire sheets, andpainting can be started immediately afterwards, or after a preliminaryphosphating treatment.

Example 5 A very thick gel is first prepared starting with:

350 gm. of water 35 gm. of bentonite This is then liquefied by adding asolution composed of:

190 gm. of water 160 gm. of sulfuric acid, 66 Baum 20 gm. of potassiumpermanganate The suspension thus formed is re-thickened byretransformation to a gel by the addition of:

200 gm. of bentonite 45 gm. of kaolin The gel thus formed is a darkviolet color and is applied on the mill scaled surface.

The change of color of the paste is manifested by its turning from darkviolet to rose or very clear brown, when the acid succeeds ininfiltrating through the pores of the calamine to the metal and producesthe liberation of hydrogen and the formation of ferrous sulfate, bothoxidized by the potassium permanganate.

The mode of application, the duration of the action and the rusting areeffected exactly as in the preceding examples.

It is to be understood that the examples given above are only by way ofillustration and not by limitation, the nature of the constituents andtheir concentrations being variable within the limits indicated in thedescription.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A method of removing mill scale from an iron body containing the sameon the surface thereof, comprising the steps of applying to said surfacea composition consisting essentially of a paste of an aqueous solutionof a non-reducing acid electrolyte having in said solution in said pastea degree of ionization greater than 0.01 ion grams per liter, saidsolution containing at least one oxidizing anion having in solution insaid paste an rH value greater than 27, the pH value of said solution inin said paste being numerically less than 2.5 at rH values between 27and 41 and being numerically up to 10 at rI-I values greater than 41,said solution being free of any acid capable of reducing the oxidizinganions in the paste, and of at least one finely divided mineralsubstance inert to said solution and being distributed therethrough anddispersed therein in an amount sufiicient to form said paste with saidsolution, said finely divided mineral substance acting as a porousvehicle for said solution so as to cause controlled oxidation of ferrousions to ferric ions and the formation of goethite which results in theloosening of the mill scale; maintaining said composition on saidsurface until said oxidizing anion is substantially completely reduced;and removing said composition after reduction of said anion, and theloosened mill scale from said surface.

2. A method of removing mill scale from an iron body containing the sameon the surface thereof, comprising the steps of applying to said surfacea composition consisting essentially of a paste of an aqueous solutionof a non-reducing acid electrolyte having in said solution in said pastea degree of ionization greater than 0.01 ion grams per liter, saidsolution containing at least one oxidizing anion having in solution insaid paste an rH value between 27 and 41, the pH value of said solutionin said paste being numerically less than 2.5, said solution being freeof any acid capable of reducing the oxidizing anions in the paste, andof at least one finely divided mineral substance inert to said solutionand being distributed therethrough and dispersed therein in an amountsufiicient to form said paste with said solution, said finely dividedmineral substance acting as a porous vehicle for said solution so as tocause controlled oxidation of ferrous ions to ferric ions and theformation of goethite which results in the loosening of the mill scale;maintaining said composition on said surface until said solution of saidcomposition reaches a pH value numerically of at least 4; and removingsaid composition after reduction of said anion and the loosened millscale from said surface.

3. A method of removing mill scale from an iron body containing the sameon the surface thereof, comprising the steps of applying to said surfacea composition consisting essentially of a paste of an aqueous solutionof a non-reducing acid electrolyte having in said solution in said pastea degree of ionization greater than 0.01 ion grams per liter, saidsolution containing at least one oxidizing anion having in solution insaid paste an rH value greater than 27, the pH value of said solution insaid paste being numerically less than 2.5 at rH values between 27 and41 and being numerically up to at rH values greater than 41, saidsolution being free of any acid capable of reducing the oxidizing anionsin the paste, and of at least one finely divided mineral substance inertto said solution and being distributed therethrough and dispersedtherein in an amount sufiicient to form said paste with said solution,said finely divided mineral substance acting as a porous vehicle forsaid solution so as to cause controlled oxidation of ferrous ions toferric ions and the formation of goethite which results in the looseningof the mill scale;-:naintaining said composition on said surface untilsaid oxidizing anion is substantially completely reduced; removing saidcomposition after reduction of said anion and the loosened mill scalefrom the surface; exposing said surface to the atmosphere until the sameis covered with rust, thereby loosening mill scale still remaining onthe surface; and removing said rust and the loosened mill scale fromsaid surface.

4. A method of removing mill scale from an iron body containing the sameon the surface thereof, comprising the steps of coating said surfacewith a composition consisting essentially of a paste of an aqueoussolution of a non-reducing acid electrolyte having in said solution insaid paste a degree of ionization greater than 0.01 ion grams per liter,said solution containing at least one oxidizing anion having in solutionin said paste an rH value greater than 27, the pH value of said solutionin said paste being numerically less than 2.5 at rH values between 27and 41 and being numerically up to 10 at rH values greater than 41, saidsolution being free of any acid capable of reducing the oxidizing anionsin the paste, and of at least one finely divided mineral substance inertto said solution and being distributed therethrough and dispersedtherein in an amount suflicient to form said paste with said solution,said finely divided mineral substance acting as a porous vehicle forsaid solution so as to cause controlled oxidation of ferrous ions toferric ions and the formation of goethite which results in the looseningof the mill scale; maintaining said composition on said surface untilsaid oxidizing anion is substantially completely reduced; and removingsaid composition after reduction of said anion and the loosened millscale from said surface.

5. A method of removing mill scale from an iron body containing the sameon the surface thereof, comprising the steps of applying to said surfacea composition consisting essentially of a paste of an aqueous solutionof a non-reducing acid electrolyte having in said solution in said pastea degree of ionization greater than 0.01 ion grams per liter, saidsolution containing at least one oxidizing anion having in solution insaid paste an rH value greater than 41, the pH value of said solution insaid paste being between 0 and 10, said solution being free of any acidcapable of reducing the oxidizing anions in the paste, and of at leastone finely divided mineral substance inert to said solution and beingdistributed therethrough and dispersed therein in an amount sufficientto form said paste with said solution, said finely divided mineralsubstance acting as a porous vehicle for said solution, said finelydivided mineral substance acting as a porous vehicle for said solutionso as to cause controlled oxidation of ferrous ions to ferric ions andthe formation of goethite which results in the loosening of the millscale; maintaining said composition on said surface until said oxidizinganion is substantially completely reduced; and removing said compositionafter reduction of said anion and the loosened mill scale from saidsurface.

6. A method of removing mill scale from an iron body containing the sameon the surface thereof, comprising the steps of applying to said surfacea composition consisting essentially of a paste of an aqueous solutionof a non-reducing acid electrolyte having in said solution in said pastea degree of ionization greater than 0.01 ion grams per liter, saidsolution containing at least one oxidizing anion having in solution insaid paste an rH value greater than 27 and being selected from the groupconsisting of Crop Cr O MnO N0 N0 1 ClO C10 1 MnOp, C10, and- IO the pHvalue of said solution in said paste being numerically less than 2.5 atrH values between 27 and 41 and being numerically up to 10 at rH valuesgreater than 41, said solutions being free of any acid capable ofreducing the oxidizing anions in the paste, and of at least one finelydivided mineral substance inert to said solution and being distributedtherethrough and dispersed therein in an amount sufiicient to form saidpaste with said solution, said finely divided mineral substance actingas a porous vehicle for said solution so as to cause controlledoxidation of ferrous ions to ferric ions and the formation of goethitewhich results in the loosening of the mill scale; maintaining saidcomposition on said surface until said oxidizing anion is substantiallycompletely reduced; and removing said composition after reduction ofsaid anion and the loosened mill scale from said surface.

References Cited in the file of this patent UNITED STATES PATENTS1,428,084 Gravell Sept. 5, 1922 1,553,881 Siegel Sept. 15, 19251,729,765 Dinley Oct. 1, 1929 2,158,992 Cook May 16, 1939 2,220,451 HuntNov. 5, 1940 2,428,804 Terry Oct. 14, 1947 2,501,145 Smith Mar. 21, 19502,554,358 Burke May 22, 1951 2,672,449 Snell et al. Mar. 15, 19542,735,818 Cardwell et al Feb. 21, 1956 OTHER REFERENCES MetallicCorrosion Passivity and Protection, Evans Edward Arnold and Co., 1948.

1. A METHOD OR REMOVING MILL SCALE FROM AN IRON BODY CONTAINING THE SAMEON THE SURFACE THEREOF, COMPRISING THE STEPS OF APPLYING TO SAID SURFACEA COMPOSITION CONSISTING ESSENTIALLY OF A PASTE OF AN AQUEOUS SOLUTIONOF A NON-REDUCING ACID ELECTROLYTE HAVING IN SAID SOLUTION IN SAID PASTEA DEGREE OF IONIZATION GREATER THAN 0.01 ION GRAMS PER LITER, SAIDSOLUTION CONTAINING AT LEAST ONE OXIDIZING ANION HAVING IN SOLUTION INSAID PASTE AN RH VALVE GREATER THAN 27, THE PH VALVE OF SAID SOLUTION ININ SAID PASTE BEING NUMERICALLY LESS THAN 2.5 AT RH VALVES BETWEEN 27AND 41 AND BEING NUMERICALLY UP TO 10 AT RH VALUES GREATER THAN 41, SAIDSOLUTION BEING FREE OF ANY ACID CAPABLE OF REDUCING THE OXIDIZING ANIONSIN THE PASTE, AND OF AT LEAST ONE FINELY DIVIDED MINERAL SUBSTANCE INERTTO SAID SOLUTION AND BEING DISTRIBUTED THERETHROUGH AND DISPOSED THEREININ AN AMOUNT SUFFICIENT TO FORM SAID PASTE WITH SAID SOLUTION, SAIDFINELY DIVIDED MINERAL SUBSTANCE ACTING AS A POROUS VEHICLE FOR SSIDSOLUTION SO AS TO CAUSE CONTROLLED OXIDATION OF FERROUS IONS TO FERRICIONS AND THE FORMATION OF GOETHITE WHICH RESULTS IN THE LOOSENING OF THEMILL SCALE; MAINTAINING SAID COMPOSITION ON SAID SURFACE UNTIL SAIDOXIDIZING ANION IS SUBSTANTIALLY COMPLETELY REDUCED; AND REMOVING SAIDCOMPOSITION AFTER REDUCTION OF SAID ANION, AND THE LOSSENED MILL SCALEFROM SAID SURFACE.